ARM64 Windows support needs tests

[jkunkee]

Now that support for ARM64 Windows ASM has been added, it should have accompanying tests.

See discussion here: #23

[jkunkee]

I started on this, but stopped when the existing Hello World tests (test/hello) fail due, I think, to an msbuild update. See:
https://github.com/jkunkee/GYP/tree/arm64-win-tests
https://github.com/jkunkee/GYP/tree/gh-pages

[refack]

I'm working bottom up, so I've added an msvs-mock test format that only generates the project files and with some tests validates their content (it does not actually try to run MSBuild).
You can run it with:

set GYP_MSVS_VERSION=2017
python gyptest.py -f msvs-mock test\actions-bare\gyptest-bare.py

example:
https://github.com/refack/GYP/blob/a1a9e19683b4f878e31af91233a35a9861b2c4e6/test/win/gyptest-link-ltcg.py#L32

[refack]

@jkunkee I've fixed MSVS testing with Python2 in latest master.

[jkunkee]

With Python 2, I'm still getting

File "C:\node\GYP\pylib\gyp\MSVS\MSVSVersion.py", line 9, in <module>
    from typing import Dict

I'm not blocked, though, since I can use Python 3.

Also, if it helps, the ARM64 compiler is x64 or x86, so the only thing testing ARM64 projects can't do is actually run the outputs.